Measurement of neutron-oxygen interaction cross section using neutron beam for diffuse supernova neutrino background search*

In 2020, Super-Kamiokande started introducing Gadolinium sulfate into the detector in order to detect neutrons more efficiently (SK-Gd) . One of the main purpose of SK-Gd is the first observation of Diffuse Supernova Neutrino Background (DSNB) using Inverse Beta Decay (IBD) events. In the SK-Gd, we can distinguish the IBD from background which do not emit neutrons thanks to high efficiency of the neutron detection.

Neutral Current Quasierastic scattering (NCQE) by atmospheric neutrinos is one of the main background of DSNB search in SK-Gd. NCQE interaction also emit neutrons, so this event cannot be distinguished from IBD by neutron detection. We have to estimate the number of NCQE events by simulation. However, the estimation has large uncertainty because the reaction of neutron with a few hundred’s of MeV via oxygen nucleus is poorly understood.

Therefore, we conducted the experiment which measure reaction of neutron and oxygen nucleus, at the Research Center for Nuclear Physics, Osaka University on October 30, and December 16, 2018. In this experiment, neutrons with 250 MeV and 30 MeV are irradiated to water target, and gamma-rays emitted from the reaction were measured. As the result of this experiment, we extracted the probability of each gamma-ray emissions. In this presentation, the results of 250 MeV experiment will be presented.